Making decision based on users' reports
If the only feature you're classifying on is the number of users making a given report, then this isn't really much to do with AI/ML. Just pick a number based on your subjective judgment and go with it.

OTOH, if you can include details of the report itself (as well as the number of reporters), I think you might be able to build a bayesian classifier that would be useful.  If you could consider location, weather, time of day, number of reporters, etc., it seems like you might be able to get something useful put together.